1. Field of the Invention
The present invention concerns the field of magnetic resonance imaging (MRI) particularly as applied in medicine for examination of patients. The present invention in particular concerns operating methods in the form of sequences for MRI in which signals that originate from nuclear spins of a specific tissue type are suppressed. For example, sequences that suppress fat tissue are among such sequences. Furthermore, the present invention concerns a magnetic resonance apparatus for implementation of such a sequence.
2. Description of the Prior Art
MR technology is a technology known for some decades with which images of the inside of an examination subject can be generated. The examination subject is positioned in a strong, static, homogeneous basic magnetic field (field strengths of 0.2 Tesla up to 7 Tesla and more) in an MR apparatus so that the subject's nuclear spins orient along the basic magnetic field. Radio-frequency excitation pulses are radiated into the examination subject to trigger nuclear magnetic resonance signals, the triggered nuclear magnetic resonance signals are detected and MR images are reconstructed based thereon. Rapidly switched magnetic gradient fields are superimposed on the basic magnetic field for spatial coding of the measurement data. The acquired measurement data are digitized and stored in a k-space matrix as complex numerical values. An associated MR image can be reconstructed from the k-space matrix populated with values by means of a multi-dimensional Fourier transformation. The temporal sequence of the excitation pulses and the gradient fields for excitation of the image volume to be measured, for signal generation and for spatial coding is thereby designated as a sequence (or a pulse sequence or measurement sequence).
In the acquisition of image data it often occurs that nuclear spins of a specific tissue type (for example fat tissue) emit a strong signal. Fat tissue is imaged very intensely in the generated images in comparison to other tissue types, such that a correct diagnosis is made more difficult. Techniques have therefore been developed in order to suppress the signal arising from fat tissue.
One of these techniques is known as “inversion recovery” (designated as IR in the following). In this technique a pulse known as an IR pulse is used that is radiated in a module known as a suppression module. The application of the IR pulse inverts the longitudinal magnetization of the nuclear spins that thereupon again approach their initial position (i.e. the alignment parallel to the B0 magnetic field) in an exponential curve with a time constant T1. This inversion can ensue slice-selectively or throughout the entire excitation pulse volume of the transmission coil.
The time constant T1 of fat tissue is shorter than the time constants of many other tissues. An acquisition module, in which the actual acquisition of the measurement data occurs, is implemented after a matching selected time duration TI (TI for “inversion time”), the longitudinal magnetization of nuclear spins of fat tissue being located precisely at the zero crossing at the point in time of the acquisition of the measurement data, such that these nuclear spins generate no signals. Such a technique is also designated as STIR (“short time inversion recovery”).
Another technique makes use of a saturation of nuclear spins of a specific type, for example of fat tissue protons. Since protons of fat tissue and of water have slightly different resonance frequencies, it is possible to excite predominantly fat tissue protons with a specific RF pulse and to thereupon destroy (dismantle) the generated signal with a gradient pulse (what is known as a spoiler gradient). This suppression of the signal of the protons of the fat tissue is often designated as a “saturation”, a term that originates from MR spectroscopy. The longitudinal magnetization of water protons is largely retained by the saturation pulse. When the data acquisition with an acquisition module ensues after a suppression module with the specific RF saturation pulse, the acquired signals originate only at a small portion of nuclear spins of the fat tissue.
Instead of signals that originate from fat tissue, signals that originate from nuclear spins of another tissue type can be suppressed with analogous techniques.
In both cases, acquisition modules that follow suppression modules are respectively executed. As is typical in MRI the acquisition of the image data often ensues via repeated execution of acquisition modules, with different parts of the measurement data are acquired in each acquisition module. In this case an IR pulse with a time offset of TI is radiated before each acquisition of the measurement data. This requires a rapid repetition of the suppression modules, for example with repetition times in the range of several tens of milliseconds.
As soon as the repetition time lies on the order of the relaxation time T1 for fat tissue or less, the state of the magnetization of the fat tissue protons changes upon each execution of a suppression module and only after a certain number of suppression modules does an equilibrium state (“steady state condition”) occur for the longitudinal magnetization of the nuclear spins of the fat tissue protons. By contrast, the longitudinal magnetization of the nuclear spins of the fat tissue protons sometimes changes very significantly from suppression module to suppression module before the appearance of the steady state condition. The image quality is thereby impaired. Among other things, the first pair of suppression modules and acquisition modules should therefore be discarded and not be used for data acquisition.
This disadvantage in particular occurs in the imaging of organs that exhibit a quasi-periodical movement (such as, for example, the lungs or the heart). In this case a PACE technique (“prospective acquisition correction”), in which the acquisition of the data is triggered by what is known as a navigator echo in order to avoid movement artifacts, is often applied to the imaging as well. Since the measurement data are for the most part acquired during movement cycles, the acquisition is triggered multiple times by a navigator echo. Given each new data acquisition during a movement cycle the problem exists that the steady state condition of the longitudinal magnetization in the fat tissue protons must first reappear. Either the measurement time is thereby significantly extended, or the suppression of signals of fat tissue protons is incomplete.
U.S. Pat. No. 5,541,514 discloses a steady state pulse sequence in which a repetition time and a flip angle are used with an alternating polarity. In the steady state condition the magnetization moves between a first value +alpha/2 and a second value −alpha/2. An RF pulse with a flip angle of alpha/2 is radiated before the beginning of the pulse sequence. A preparation of the nuclear spins is hereby achieved and the steady state condition occurs faster.
A sequence called TOMROP (“T One by Multiple Read Out Pulses”) is described in the article by Brix et al., “Fast and precise T1 imaging using a TOMROP sequence”, Magn. Reson. Imaging, 1990; 8(4):351-356. A train of interrogation pulses with small flip angles β generates a set of gradient echoes. A steady state condition for the longitudinal magnetization appears during this train. Before the train of interrogation pulses a selective pulse known as an α-pulse shifts the longitudinal magnetization into a defined initial state. The longitudinal magnetization develops differently dependent on the flip angle α.